Introduction: Natural killer (NK) and natural killer T (NKT) cells provide a first line of defense against infection. However, these cells have not yet been examined in patients with Lyme arthritis, a late disease manifestation. Lyme arthritis usually resolves with antibiotic treatment. However, some patients have persistent arthritis after spirochetal killing, which may result from excessive inflammation, immune dysregulation and infection-induced autoimmunity. Methods: We determined the frequencies and phenotypes of NK cells and invariant NKT (iNKT) cells in paired peripheral blood (PB) and synovial fluid (SF) samples from eight patients with antibiotic-responsive arthritis and fifteen patients with antibiotic-refractory arthritis using flow cytometry and cytokine analyses. Results: In antibiotic-responsive patients, who were seen during active infection, high frequencies of CD56bright NK cells were found in SF, the inflammatory site, compared with PB (P <0.001); at both sites, a high percentage of cells expressed the activation receptor NKG2D and the chaperone CD94, a low percentage expressed inhibitory killer immunoglobulin-like receptors (KIR), and a high percentage produced IFN-γ. In antibiotic-refractory patients, who were usually evaluated near the conclusion of antibiotics when few if any live spirochetes remained, the phenotype of CD56bright cells in SF was similar to that in patients with antibiotic-responsive arthritis, but the frequency of these cells was significantly less (P = 0.05), and the frequencies of CD56dim NK cells tended to be higher. However, unlike typical NKdim cells, these cells produced large amounts of IFN-γ, suggesting that they were not serving a cytotoxic function. Lastly, iNKT cell frequencies in the SF of antibiotic-responsive patients were significantly greater compared with that of antibiotic-refractory patients where these cells were often absent (P = 0.003). Conclusions: In patients with antibiotic-responsive arthritis, the high percentage of activated, IFN-γ-producing CD56bright NK cells in SF and the presence of iNKT cells suggest that these cells still have a role in spirochetal killing late in the illness. In patients with antibiotic-refractory arthritis, the frequencies of IFN-γ-producing CD56bright and CD56dim NK cells remained high in SF, even after spirochetal killing, suggesting that these cells contribute to excessive inflammation and immune dysregulation in joints, and iNKT cells, which may have immunomodulatory effects, were often absent.